Osteosynthesis plates for stabilization of bone fractures are typically applied with bone screws. Traditionally, bone screws were used to compress a plate onto the bone surface to provide stable fixation. More recently, locking plates have been introduced, which typically have threaded receiving holes for positive, angle-stable engagement with the threaded head portion of a locking screw. These types of locking plates have been described in French patent document 742,618; U.S. Pat. No. 5,709,686; and U.S. Pat. No. 5,741,258. These locking plates can provide more stable fixation in the ends of weak, osteoporotic bone compared to traditional, non-locking plates.
Clinically, locking plate constructs face two principal challenges that may be addressed by the present invention. First, a locked plate construct may alter the load distribution in bone, which may either cause bone resorption in case of load shielding, or bone fracture due to implant-induced stress risers. Second, the high stiffness of a locked plate construct can suppress relative displacement between bone fragments, whereby this interfragmentary motion is important to promote the natural cascade of fracture healing by callus formation. Therefore, overly stiff locking plate constructs may delay or prevent fracture healing by callus formation.
The present invention addresses both of these challenges. First, elastic suspension of the receiving holes relative to the osteosynthesis plate promotes load distribution between the screws that connect a bone segment to the plate, thereby reducing stress risers and load shielding effects. Second, elastic suspension of the receiving holes relative to the osteosynthesis plate enables small, controlled amounts of relative motion between bone fragments connected by the plate. These controlled amounts of relative motion can promote fracture healing by callus formation.
U.S. Pat. No. 4,943,292 describes the use of a cushion of elastic material between the screw head and the bone plate to reduce construct stiffness and to allow early motion at the fracture site. However, U.S. Pat. No. 4,943,292 can only be practiced with non-locking screws, whereby the plate has to be compressed onto the bone surface. This prevents relative motion between the plate and the bone surface required to induce axial motion at the fracture site.
To enable relative motion between the plate and the bone surface with non-locking screws, the use of resorbable or biodegradable materials spaced between a screw head and a plate (US 2010/0249850; U.S. Pat. No. 4,338,926) has been disclosed. However, this prior art provides relative stiff fixation in the early healing phase where a flexible construct is most desirable to stimulate formation of a healing callus. Furthermore, the prior art progressively loosens over time as resorption of the biodegradable material at the screw-plate interface causes an increasing lack of fixation stability.
U.S. Patent Application Publication No. US2011/0118742A1 describes means for permanent displacement of a screw hole relative to a bone plate to generate static compression across the bone fracture spanned by the bone plate. Their invention relies on plastic deformation of the screw hole, and is therefore not suitable for elastic suspension of a screw hole to induce dynamic motion at a fracture site to stimulate fracture healing by callus formation. Moreover, deformation of a locking screw hole may compromise the screw-bone interface.
The present invention employs elastic suspension of receiving holes in a plate, whereby the screw hole does not undergo deformation, and whereby said receiving holes are threaded to receive locking screws that enable plate elevation over the bone surface. Alternatively, elastically suspended receiving hole elements may extend past the lower surface of the plate when used with non-locking screws to allow plate suspension over the bone surface, which is required to support relative motion between the plate and the bone surface.
It is therefore beneficial and desirable to stabilize a bone fracture with the plate of the present invention to enhance load distribution between screws, and to promoted fracture site motion when fracture healing by callus formation is desired.